Atomic Composition
What everything around you is actually built from
How this deck works. Every slide is colour-coded: Read for the explanation, Look for diagrams and models, Do for experiments and simulations, and Check for questions. Move with the buttons at the bottom, or the ← → keys on a keyboard.
You have been saying "molecule" for a whole topic. Time to open one up.
Where you left off · Topic 1
In Thermal Energy you tracked how particles move: locked in a grid in a solid, sliding in a liquid, flying free in a gas. Faster motion, more thermal energy. You never once asked what those particles are built out of.
That is this topic. Every molecule you watched vibrate is assembled from smaller pieces called atoms, and here is the strange part: there are only about 118 different kinds of atom in the entire known universe. Around 90 of them occur naturally on Earth.
Everything you have ever touched — your phone, the ocean, your own body, a distant star — is those same few dozen building blocks, arranged differently.
What an atom is made of
An atom is the smallest piece of a pure substance that still counts as that substance. Split a carbon atom and you no longer have carbon.
Atoms are almost unimaginably small. Line up a few million of them side by side and you would just about cross the head of a pin.
Three parts, two locations:
- Protons — positively charged, packed in the center.
- Neutrons — no charge, packed in the center alongside the protons. Together the protons and neutrons form the nucleus.
- Electrons — negatively charged, extremely light, moving in the huge space surrounding the nucleus.
The nucleus holds nearly all of the atom's mass but takes up almost none of its volume. If an atom were the size of a football stadium, the nucleus would be about the size of a marble at the center — and the rest would be the space where the electrons are found.
The number of protons is the atom's ID number. It is called the atomic number, and it decides everything about which element the atom is. Six protons means carbon. Always. Add a seventh and it is not carbon anymore — it is nitrogen.
A model of a carbon atom
Not a photograph — a model. Drawn to make the parts visible, not to scale.
Teacher note · common misconception
The neat rings in this drawing are a teaching model, not a picture of reality. Electrons do not travel on tidy circular tracks like planets. Say out loud that this is a model built for human eyes, and that scientists picture electrons as occupying fuzzy regions of probability. Students who hear this in grade 7 have a far easier time in chemistry later.
Test yourself
An atom has 8 protons, 8 neutrons and 8 electrons. A second atom has 8 protons, 10 neutrons and 8 electrons. What is true?
An element is one kind of atom, and nothing else
An element is a pure substance built from only one kind of atom. A copper wire is nothing but copper atoms. A helium balloon is nothing but helium atoms. There is no ordinary chemical way to break an element down into something simpler — that is what makes it an element.
Every element gets a one- or two-letter symbol. The first letter is always capital; a second letter, if there is one, is always lowercase. C is carbon. Co is cobalt. CO — capital C, capital O — is not an element at all; it is carbon monoxide, two different atoms stuck together. Capitalization is not decoration here. It carries meaning.
Some symbols look like they do not match the name, because they come from Latin:
- Na — sodium, from natrium
- K — potassium, from kalium
- Fe — iron, from ferrum
- Au — gold, from aurum
- Pb — lead, from plumbum (the same root as "plumbing")
Teacher note · common misconception
Coming out of Topic 1, students often believe heating a substance makes its atoms bigger. It does not. Heating increases how far and how fast atoms move; the atoms themselves are unchanged. A steel bridge expands in July because its atoms vibrate over a wider range and take up more room collectively — not because each atom swelled.
The first 20 elements
The periodic table is a filing system. Elements are placed in order of atomic number, and neighbors in a column behave alike.
Reading it is simple once you know the pattern. Hydrogen is 1 because it has one proton. Carbon is 6. Oxygen is 8. Calcium is 20. The number is a count, not a ranking.
The far-right column is worth knowing: helium, neon and argon are the noble gases. They almost never combine with anything, which is exactly why we fill balloons and light bulbs with them.
Test yourself
Which of these is an element?
Build it and shake it
Pick a substance. Then use the slider to add energy — the same energy you controlled in Topic 1.
Things worth noticing as you click through:
- O₂ has two atoms but it is still an element — both atoms are the same kind. "More than one atom" does not automatically mean compound.
- H₂O and CO₂ both contain oxygen, and they could hardly be more different. What an atom is bonded to changes everything.
- NaCl does not draw as a single molecule, because it is not one. Salt is a repeating three-dimensional grid of sodium and chlorine.
- Turn the energy up on any of them: the atoms jiggle harder but the bonds hold and the formula never changes. Adding thermal energy alone does not rearrange atoms — that takes a chemical reaction, which is Topic 3.
Molecules, compounds, and how to read a formula
When atoms bond together they form a molecule. If the bonded atoms are different kinds, the substance is also a compound.
A chemical formula is a shorthand for exactly which atoms are in there and how many of each. The small number written below and after a symbol — the subscript — counts the atoms of the symbol it follows. No subscript means one.
H₂O — water
2 hydrogen atoms, 1 oxygen atom. Total: 3 atoms.
CH₄ — methane
1 carbon atom, 4 hydrogen atoms. Total: 5 atoms. This is natural gas.
CO₂ — carbon dioxide
1 carbon atom, 2 oxygen atoms. Total: 3 atoms. You exhale it constantly.
C₆H₁₂O₆ — glucose
6 carbon, 12 hydrogen, 6 oxygen. Total: 24 atoms. This is the sugar your cells run on.
A compound is not a blend of its ingredients — it is a brand-new substance with its own properties. Sodium on its own is a soft metal that reacts violently with water. Chlorine on its own is a poisonous green gas. Bond them and you get sodium chloride: the white crystals you sprinkle on french fries.
Teacher note · common misconception
Watch for students reading H₂O as "H two O atoms" and concluding there are two of everything. Also introduce the difference between a subscript and a coefficient early: 2H₂O means two separate water molecules, six atoms in total. Getting this straight now prevents real trouble when you balance equations in Topic 4.
Test yourself
How many atoms are in one molecule of C₆H₁₂O₆ (glucose)?
Pull the iron out of your breakfast
Is there really a metal element in your cereal?
Materials
- Iron-fortified cereal (label should say 45%+ iron)
- A strong magnet
- A zip bag
- Warm water
- A rolling pin or heavy cup
- A white plate
Steps
- Read the nutrition label first and write down the % Daily Value for iron. Higher is better for this test.
- Put about a cup of cereal in the zip bag, press the air out, seal it, and crush it into a fine powder.
- Add roughly a cup of warm water, seal again, and squeeze the bag until you have a smooth grey slurry. Let it sit 15 minutes.
- Lay the bag flat. Hold the magnet against the outside of the bag and drag it slowly through the slurry, in one direction, for about a minute.
- Keeping the magnet pressed to the plastic, lift that spot of the bag up to a window or lamp and look closely at the plastic right under the magnet.
You should see tiny black specks gathering and moving with the magnet. That is elemental iron — Fe, atomic number 26.
Cereal makers add extremely fine iron powder during production, because iron is an element your body cannot manufacture and must eat. Your red blood cells use it to carry oxygen. It is not a substitute or an imitation: it is the same element as a nail or a skillet, ground fine enough to swallow safely.
This is also a demonstration of a mixture. The iron was never chemically bonded to the cereal, which is exactly why a magnet — a physical tool, not a chemical one — can pull it back out.
Teacher note
Refrigerator magnets are usually too weak. A neodymium magnet from a hardware store, or the magnet out of an old speaker, makes the difference between a dramatic result and a disappointing one. Cereals marketed as high-iron work best; a 100% Daily Value cereal gives the clearest specks.
Make salt reappear
Compound or mixture? Let evaporation decide.
Materials
- Table salt
- Sugar
- Warm water
- 2 dark plates or saucers
- 2 spoons
- A sunny windowsill
Steps
- Dissolve half a teaspoon of salt in about two tablespoons of warm water. Stir until the water looks completely clear.
- In a second cup, do exactly the same with sugar.
- Pour a thin layer of each solution onto its own dark plate. Label them.
- Leave both plates on a sunny windowsill for 1–2 days. Do not touch them.
- When the plates are dry, look closely — a phone camera zoomed in works well — and compare what is left behind on each plate.
Both plates end up with solid crystals. The salt forms small, hard cubes; the sugar forms larger, more irregular clumps.
Dissolving is a physical change, not a chemical one. The salt particles spread out among the water molecules and became too small to see, but they never stopped being salt. Salt water is a mixture, and the water leaving proves it: a physical process separated the two ingredients and gave you the salt back unchanged.
Now compare that with the water itself. Evaporating, freezing, or boiling water will never yield a pile of hydrogen and a pile of oxygen, because water is a compound and its atoms are chemically bonded. Separating those takes a chemical reaction — the subject of your next topic.
Teacher note · common misconception
"It dissolved, so it disappeared" is extremely common and it quietly undermines conservation of mass in Topic 4. The dark plate matters — white crystals on a white saucer are nearly invisible and students conclude nothing happened. Weighing the covered cup before and after dissolving makes an excellent extension.
Element, compound, or mixture?
Three categories students mix up constantly. The difference is visible at the particle level.
The practical test: can you separate it without a chemical reaction? You can boil ocean water and get salt and water back, so ocean water is a mixture. No amount of boiling, filtering or straining will get hydrogen and oxygen out of water, because water is a compound.
Now sort these
Tap a button. You get the reason immediately. Three of these are traps.
Test yourself
Oxygen gas has the formula O₂ — two atoms bonded together. Is it an element or a compound?
Eight words you now own
Tap any card to flip it. Tap again to flip it back.
Where you already use this
The recycling bin on your curb
Aluminum cans are close to pure element 13. Recycling one takes roughly 5% of the energy needed to pull fresh aluminum out of ore, which is why aluminum is the most valuable thing in that bin.
The wiring inside your walls
Copper, element 29. It is used almost everywhere in American homes because copper atoms let electrons move through them with very little resistance.
A neon sign in a diner window
Pure neon glows red-orange when electricity runs through it. The blues and greens in those signs are other noble gases, or ordinary glass tubes with colored coatings.
Your cereal box nutrition label
Iron, calcium, zinc, potassium — that column is a list of elements. Your body needs specific atoms it cannot build for itself.
The chlorine at a community pool
Element 17, added as compounds that release it into the water, where it destroys bacteria. That summer pool smell is chlorine chemistry at work.
Helium at a birthday party
Element 2. It floats because a helium atom is far lighter than the nitrogen and oxygen molecules in air, and it is safe indoors because noble gases refuse to react — unlike the hydrogen that ended the airship era.
Test yourself
A student stirs sand into water and says "now it's a compound." What is the best correction?
Topic 2 quiz — 8 questions
One try per question. You get the reasoning right after you answer.
Up next · Topic 3
Chemical Reactions. So far, nothing you have done has changed a substance into a different one. Melting ice, dissolving salt, warming a gas — the atoms stayed bonded exactly as they were. Next topic, the bonds break and re-form, and you get substances that were not there before.